This invention involves multistable cholesteric liquid crystal displays exhibiting ultra fast response times with both video speed and gray scale capability.
The rapid developments in high-density data acquisition and storage continue to create demands for higher speed, higher information content display devices. Additionally, technological developments, particularly those involving video displays, continue towards a decreased reliance on relatively bulky and high power consumption cathode ray tube (CRT) devices. Instead, emphasis is being placed on the construction of smaller, lighter and overall more compact systems such as those being developed by the flat panel display (FPD) industry. Certainly, the most active component of the FPD industry is that area based on the use of liquid crystals to provide the necessary light modulation and storage.
Although flat panel liquid crystal displays have already achieved widespread use, there are still major drawbacks associated with these displays. For example, most high information content displays require use of polarizers and other light attenuating components thus creating relatively high power consumption back lighting requirements. This is a severe disadvantage in many applications, such as in operation of portable notebook type displays. There are additional disadvantages associated with the two most commonly used liquid crystal FPDs which employ either active matrix thin film transistors (AMTFT) or supertwist nematic (STN) technologies. For example, AMTFT devices use transistors for each pixel to provide memory effects. In addition, they also require expensive, ultra high resistance liquid crystal materials to minimize RC losses which in turn enables longer holding times. AMTFT displays are both difficult and costly to produce and, at present, are limited to relatively small size displays. Additionally, they are not zero field image storage systems as they require a constant power input for image refreshing. The STN displays do not possess inherent pixel gray scale capability as a result of the extreme steepness of the optical voltage response curve of the liquid crystals employed. It should be noted that STN displays have an inherent slow response rate. Their response rates are too slow for video applications. Finally, it should also be mentioned that both AMTFT and STN displays have very restricted viewing angles.
An important advance in FPD technology would be development of a display which eliminates the need for light modulating components (e.g., polarizers, retarders, analyzers, color filters, etc.) and thus the need for high power consumption back lighting. At the same time, it is understood that this elimination should not compromise other display properties. An attractive potential candidate in this regard would be use of cholesteric liquid crystals to replace the nematic and super-twist nematic liquid crystal mixes currently employed. It has long been known that cholesteric LCDs can provide light modulation without recourse to polarizers and back lighting requirements. This light modulation capability arises from the ability of cholesteric liquid crystals to exist in either a reflective or in a light scattering structure. In the reflective state, the liquid crystal molecules are arranged in domains with the long axes of the molecules roughly parallel to each other in each hypothetical layer. However, steric and asymmetry properties of the molecules result in a progressive slight displacement of the long axis of the molecules with respect to adjacent layer. The combined net effect of these small displacements is creation of a helical molecular structure in each domain. The helical axis is the director of the domain. When the domain directors are roughly parallel to each other and perpendicular to the cell surface, electromagnetic radiation perpendicularly incident on the LCD cell surface is efficiently transmitted except for a relatively narrow wavelength band which is reflected. The wavelength of the reflected radiation is given by the relationship lambda (.lambda.)=n P where n is the average refractive index (n=(n.sub.e +n.sub.o)/2) [n.sub.e is the extraordinary refractive index; n.sub.o is the ordinary refractive index] and P is the pitch (i.e., twice the repetition length of the helical structure) of the liquid crystals. The reflected wavelength maximum is selectable by appropriate adjustment of the n and/or P values of the liquid crystal mixes employed. This is referred to as the Reflective State. If the selected wavelength is outside the visible region of the electromagnetic spectrum (e.g., in the infrared), the reflective cholesteric liquid crystal texture would be described as a transmissive state.
In contrast with the visible reflective or transmissive state, the scattering structure represents a 2-D random orientation of the helical axis of the domains. This randomization provides an efficient scattering of incident electromagnetic radiation. If the thickness of the liquid crystal medium is sufficiently large, and the pitch, birefringence of the liquid crystals, and size of domain are satisfactory, the majority of incident visible radiation is scattered and the display appears to be milky white. If, however, in contrast, the thickness of liquid crystal medium is relatively thin (e.g., less than 5 microns), only a small percentage of the incident radiation will be back scattered with the remainder being transmitted. If the liquid crystal display has been assembled using a back plate liner which can strongly absorb visible radiation, the display will appear to be black when the liquid crystals are in a light scattering texture. This is referred to as the Dark State.
Intermediate orientation of domains, in which the domain directors are neither perpendicular to the surface nor parallel to the surface, but at an orientation in between. There are infinite stable states between the Reflective State and Dark State. These are referred to as intermediate states which are capable of displaying infinite "gray shales."
Switching from any state to another is achieved by application of an appropriate electrical field impulse thus providing the desired light modulation.
Although cholesteric LCDs offer potential advantages in terms of avoiding the need for light modulating components, there has been no commercial development involving these displays in direct view mode applications. Perhaps the most important reason for this lack of commercial development is the relatively poor contrast ratio typically available when these displays are operated in a direct view mode using the difference in transmitted and scattered light. This arises from the fact that the diffuse light scattering texture permits transmission of a significant portion of the incident visible light at the cell gap typically employed in LCDs. This results in a reduction of the contrast ratio. An additional disadvantage of cholesteric LCDs is that they require significantly higher driving voltages than many other LCDs.
It has been found that a particularly important application of the present invention is achieved using cholesteric liquid crystal displays which exhibit multistable zero field stability, specifically ones in which this zero field stability is achieved without recourse to polymer gel additives. In these displays, the cholesteric LC domains helical axis exhibit a continuous orientation distribution of states ranging from reflective to dominantly light scattering in which each intermediate state is indefinitely stable under zero field conditions. Establishment of this zero field multistability is achieved by minimizing the interactions (i.e., boundary effects) existent between the solid substrates and the liquid crystal domains. Typically these solid surface-LC interactions result in establishment of a preferred liquid crystal domain texture in the absence of any applied fields. For example, these interactions result in slow zero field relaxation of light scattering domains to reflective domain structures. These boundary driven relaxation phenomena have frustrated efforts by earlier inventors to create permanently stable zero field bistable displays (e.g., U.S. Pat. Nos. 3,707,331; 3,821,780 and 3,806,230). However, it has been found that zero field multistable cholesteric displays can be achieved by elimination of any alignment layer or directional rubbing of the solid substrates used to encase the LC mix. The elimination of the directive surface interaction reduces the system energy difference between the most reflective domain orientation and the various other domain orientations. In this manner, zero field multistability is achieved as the most reflective liquid crystal structure is of essentially the same energy as the darkest light scattering array and this energy equalization includes all intermediate domain structures spanning the most reflective and darkest light scattering extremes. The zero field domain structures of a particular texture can be varied using a very short duration high voltage pulse or a lower voltage pulse of longer duration thus providing a continuous zero field gray scale capability. The progressive change in domain orientations can be realized starting with either the reflective structure extending all the way to the darkest light scattering state, or conversely, starting with the darkest light scattering state and extending all the way to the most reflecting structure. Each intermediate state so established between the reflective and light scattering extremes is indefinitely stable under zero field conditions.
Although the present invention focuses on applications involving multistable zero field cholesteric liquid crystal displays, those schooled in the art of LCD technology will recognize that the invention described herein is applicable to numerous other liquid crystal display types.